entries into register won't record

[gnua at sundew.org.uk]

This seems to happen at unpredicatable times.  When entering a sequence 
of transactions, tonight it was the second one, having the transfer 
account auto-completed, and entered the amount, pressing enter won't 
record the entry.  Pressing the record button has the same effect.  
Closing the account, and forcing save changes does not save the entry.  
I've re-entered the entry, changed to transaction journal and I can 
enter and move onto a new, blank entry as I would expect.  What am I 
doing wrong? !

Thanks,
Andy